1. Field of the Invention
The present invention generally relates to methods and systems for generating and detecting terahertz radiation. More particularly, the present invention relates to detecting fluorescence created by the interaction of plasma and terahertz radiation.
2. Description of Related Art
The work of Zhang, et al. as exemplified by U.S. Pat. Nos. 7,531,802; 7,595,491; 7,652,253; and 7,808,636, among others, underscores the unique potential for terahertz (THz) wave sensing and detection to provide marked improvements in the detection and analyses of materials, in particular, materials harmful to humans, such as, explosives. As is known in the art, THz electromagnetic radiation lies in the electromagnetic spectrum between infrared radiation waves and microwaves.
Although THz-based detecting techniques continue to excel in spectroscopic studies, such as, in-situ and non-destructive evaluation of materials, the ability to conduct such measurements from a significant remote or “stand-off” distance has remained elusive. This is primarily due to inherent THz absorption by atmospheric water vapor. Various THz detection methods have been introduced; however, methods for true remote coherent detection remain to be developed. Recently, the ability to remotely generate THz using optical wavelengths focused at a distance has shown promise in reducing THz free space interaction by a factor of two. Nonetheless, this does not entirely solve the problem since THz attenuation in air can be as high as 100 decibels/meter. Among various THz detection methods, it has been demonstrated that air may be used as a coherent THz detector, and applying a modulated electric field to the nonlinear optical interaction between a THz pulse and an 800 nm optical pulse can greatly enhance the detected coherent information. However, methods for true remote coherent detection without the need for on-site electrodes or cabling remain to be developed.
Laser-induced plasma continues to gain interest because of its increasing number of scientific and technological applications in photo-ionization, high-harmonic generation, laser-induced breakdown spectroscopy, spark-induced breakdown spectroscopy, and the generation and detection of broadband terahertz (THz) pulses. Since the advent of millijoule, femtosecond pulsed lasers, laser-induced plasma has been employed to study the interaction between light and matter and reveal ultrafast dynamics of solids, liquids, and gases.
While some work has been conducted to send THz waves to a remote distances directly, these methods are limited by the extreme absorption of THz by atmospheric water vapor and therefore cannot be extended beyond a few meters, that is, distances which still make the, for example, hazard under study a potential threat to the operator.
Aspects of the present invention provide methods and systems introducing new tools to the arsenal for remote detection, for example, for biological, physical, and defense-based applications, among others.